The present invention relates to an A/D converter (analog-to-digital converter), and to a self-diagnosis technique for an A/D converter installed in a Micro Controller Unit (hereinafter referred to as the MCU) that requires high safety for exerting vehicle control, for example.
Recent automobiles have various functions, and have an in-vehicle system installed for realizing those functions. On the other hand, since an occurrence of a failure in an automobile is life-threatening, the in-vehicle system is strongly required to be highly reliable.
Such an in-vehicle system is equipped with many analog output sensors, and system control is exerted using the output values of the sensors. For example, in connection with an airbag, an analog output sensor converts acceleration or pressure into an analog signal, and inputs the analog signal to the MCU. The MCU periodically performs AD (analog-to-digital) conversion to the received analog signal, and determines whether or not a car collision has occurred based on the AD conversion result.
As in the example of the airbag described above, the analog voltage output from the analog output sensor is generally acquired as digital data through use of the A/D converter installed in the MCU. Therefore, since a failure of the A/D converter in the in-vehicle system may possibly invite a fatal error in the system, it is demanded to surely carry out failure detection as to the A/D converter.
As to the A/D converter, the successive approximation A/D converter is widely used. The successive approximation A/D converter includes a D/A converter, and performs conversion by comparing an analog input signal and a feedback signal of the D/A converter with each other by 1 bit. That is, when the D/A converter is suffering from a failure, the A/D converter does not output a correct AD conversion result, and hence the system as a whole cannot operate correctly.
Japanese Unexamined Patent Application Publication No. 2009-71459 (hereinafter referred to as “Takamatu et al.”) discloses a technique for detecting a failure of a D/A converter. According to the technique of Takamatu et al., firstly, digital data corresponding to a desired analog voltage to be output from the D/A converter is input to the D/A converter being the self-diagnosis target, and an analog voltage output from the D/A converter is subjected to AD conversion by the A/D converter installed in the MCU. Then, the acquired digital data is compared against the digital data having been input to the D/A converter being the self-diagnosis target, and whether or not the difference is appropriate is checked.
With the technique of Takamatu et al., the D/A converter built in the A/D converter is used for diagnosing a failure. In this case, when the A/D converter is performing normal AD conversion, a failure diagnosis cannot be performed using the D/A converter. In addition, in the case where a failure diagnosis is performed using the D/A converter when the A/D converter is performing normal AD conversion, in addition to the A/D converter, a separate failure diagnosis-dedicated A/D converter becomes necessary. This increases the circuit scale.